Magnetic sensors find application in various areas of science and technology. Persistent efforts have led to the development of new, highly sensitive magnetic sensors as well as the improvement of existing technologies. Useful background information can be found in the following references:                Hari, R. & Salmelin, R. Magnetoencephalography: From SQUIDs to neuroscience: Neuroimage 20th Anniversary Special Edition. NeuroImage 61, 386-396 (2012);        Gaffney, C. Detecting Trends in the Prediction of the Buried Past: A Review of Geophysical Techniques in Archaeology. Archaeometry 50, 313-336 (2008);        Drung, D. et al. Highly Sensitive and Easy-to-Use SQUID Sensors. IEEE Trans. Appl. Supercond. 17, 699-704 (2007);        Dang, H. B., Maloof, A. C. & Romalis, M. V. Ultrahigh sensitivity magnetic field and magnetization measurements with an atomic magnetometer. Appl. Phys. Lett. 97, 151110 (2010);        Ripka, P. & Janosek, M. Advances in Magnetic Field Sensors. IEEE Sens. J. 10, 1108-1116 (2010)].        
Improving sensitivity has been a strong motivation for development of subfemtotesla magnetometers. However, due to the 1/r3 decay of magnetic dipolar fields, sensor size is a critical further parameter. Consequently, a number of approaches are striving for high sensitivity in combination with reduced sensor sizes:                Rugar, D., Yannoni, C. S. & Sidles, J. A. Mechanical detection of magnetic resonance. Nature 360, 563-566 (1992);        Degen, C. L., Poggio, M., Mamin, H. J., Rettner, C. T. & Rugar, D. Nanoscale magnetic resonance imaging. Proc. Natl. Acad. Sci. 106, 1313-1317 (2009)        Huber, M. E. et al. Gradiometric micro-SQUID susceptometer for scanning measurements of mesoscopic samples. Rev. Sci. Instrum. 79, 053704 (2008);        Balasubramanian, G. et al. Nanoscale imaging magnetometry with diamond spins under ambient conditions. Nature 455, 648-651 (2008).        
FIG. 1d compares magnetic field sensitivities and characteristic sizes for various implementations restricted to room-temperature sample and far field techniques. In essence, the graph shows that small sensors with subpicotesla sensitivity have not been realized prior to the present invention.
The favourable material properties of diamond as well as the optical and spin properties of nitrogen vacancy (NV) defect centres allow for optical polarization, manipulation and readout of its spin state [Doherty, M. W. et al. The nitrogen-vacancy colour centre in diamond. Phys. Rep. 528, 1-45 (2013)]. This opens new ways for the implementation of robust solid state sensors for a variety of quantities [Acosta, V. M. et al. Temperature Dependence of the Nitrogen-Vacancy Magnetic Resonance in Diamond. Phys. Rev. Lett. 104, 070801 (2010); Doherty, M. W. et al. Measuring the defect structure orientation of a single NV-centre in diamond. New J. Phys. 16, 063067 (2014)]. In particular as magnetic field sensors, NV-based approaches offer opportunity for detection of magnetic field signals both with high spatial accuracy (nanometer) as well as high field sensitivity [Maze, J. R. et al. Nanoscale magnetic sensing with an individual electronic spin in diamond. Nature 455, 644-647 (2008)]. In addition to the utilization of individual electronic spins in diamond material, it is also known to utilize ensembles of NV centers such as described in Acosta, V. et al. “Diamonds with a high density of nitrogen-vacancy centers for magnetometry applications” Phys. Rev. B 80, 115202 (2009). While this approach sacrifices the potential atomic scale resolution of single spin magnetometers it has the potential of gaining higher field sensitivity with still smaller sensor dimensions than e.g. atomic vapour-based designs.
Magnetic field detection is based on ground state Zeeman shifts of spin sublevels of NV centres ΔE=γℏB, where γ is the gyromagnetic ratio of the electron spin and B is the field to be measured. ΔE is best determined by exploiting coherent control of the electronic spin state of the NV centres in its ground state (FIG. 1a-b). In essence the spin acquires a phase φ=γ·B·Tφ during sensing time Tφ (B is the averaged field) in Ramsey or spin echo-type measurements [see, for example, Taylor, J. M. et al. High-sensitivity diamond magnetometer with nanoscale resolution. Nat. Phys. 4, 810-816 (2008)]. Optical excitation with a laser pulse concludes a single field evaluation step by invoking spin state dependent fluorescence and reinitializing the spin state via the spin selective singlet decay of the NV centers. In essence, the fluorescence response of the system S is modulated with sin(B).
It is an aim of embodiments of the present specification to enhance the sensitivity of magnetic field measurements with ensembles of spin centres and in particular to provide a magnetometer which has an improved combination of small size and high sensitivity (i.e. lower magnetic field strength measurement capability) when compared to prior art magnetometers as illustrated in FIG. 1d. 